As is well known, a typical semiconductor controlled rectifier comprises a semiconductor substrate having first, second, third and fourth layers of alternate n- and p-types of conductivities. One main surface of the semiconductor substrate is formed of the exposed surfaces of the first and the second layers and the other main surface is formed of the exposed surface of the fourth layer. A cathode electrode is disposed in ohmic contact with the first layer on one main surface and a gate electrode is disposed in ohmic contact with the second layer on the same surface as mentioned above. On the other hand, an anode electrode is formed in ohmic contact with the fourth layer on the other main surface. An example of a semiconductor controlled rectifier device that employs amplifying gate structure may be found in the U.S. Pat. No. to Kimura et al. 4,063,270.
If a voltage in the forward direction is applied across the anode and the cathode of such a semiconductor controlled rectifier and, further, if a pulse-like gate signal is applied across the gate and cathode, electric current flows between the anode and the cathode rendering the semiconductor controlled rectifier conductive. The conversion of the semiconductor controlled rectifier from the cut-off state to the conductive state is referred to as "turn on".
If the initial firing, which causes the semiconductor controlled rectifier to be turned on, takes place in a narrow region, a heavy firing current flows through the firing region resulting in an increase in temperature and producing a so-called hot spot. Such a phenomenon develops conspicuously particularly when the rate of current increase di/dt (where i denotes a current flowing between the anode and the cathode) flowing into the semiconductor controlled rectifier device is high. The reason is because when di/dt is great, large switching power is dissipated causing the temperature of the narrow firing region to be increased. The development of the hot spot presents the probability of damage to the semiconductor controlled rectifier device, thereby greatly decreasing the reliability of the device.